13. Colliding worlds

Impacts with the Earth

The solid surfaces of almost all planets and satellites, from Mercury and the Moon to Jupiterís satellite Ganymede, are marked with impact craters; the one exception is Jupiterís moon Io whose volcanic outpourings of lava have erased all the craters from its surface. The impact craters are the scars of past collisions with cosmic objects speeding through space. The terrestrial planets originated by the coalescence of these objects, and one of them tore enough material out of our planet to forge the Moon. Large comets or asteroids might even have brought water to the young Earth. And the cosmic barrage continues today. A hail of cosmic objects is now pelting the Earth as it sweeps through space. Some of them are tiny, and burn up in the atmosphere. Every month at least one house-sized object is blowing up when it enters our air, producing a blast as forceful as a nuclear bomb. Now and then a bigger one gets through, gouging a crater out of the ground and even threatening the inhabitants of Earth.

Explosions in the atmosphere

The largest object to strike the Earth in the 20th century wasnít quite big enough to reach the ground. It disintegrated between 5 thousand and 10 thousand meters up, over the Podkamennaya Tunguska River in central Siberia. The shock wave generated by the ensuing explosion leveled trees over 2 trillion (2 x 1012) square meters of the underlying land, an area larger than New York City and surrounding suburbs. The energy produced was equivalent to the aerial explosion of the nuclear bomb that leveled Hiroshima. So much devastation, yet it failed to produce a crater.

The terrestrial impact record

Even the largest craters, produced by the biggest comets or asteroids, will gradually disappear from sight with the passage of time. The same forces that erode mountains, deposit sediments, eject lava and shift continents are erasing the craters and removing them from sight. If not for these dynamic forces, the craters accumulated over the ages would be as densely distributed and prominent as the overlapping craters on the Moon.

Only about 160 terrestrial impact craters have managed to survive the ravages of time. They have been identified on images taken from space, using airplanes, the Space Shuttle, or satellites such as Landsat. These craters can be first identified from aerial photographs, by their circular shapes and uplifted and overturned rims. But since other processes, such as volcanism and erosion, can also leave circular holes, confirming evidence of an impact origin must be gathered from rocks in and around the crater.

How do geologists know that some terrestrial craters are due to the explosions of projectiles coming from space? They look for rocks that have been transformed under the conditions of extreme temperature, pressure, and shock associated with a high-velocity, external impact. The most apparent shock effect is the formation of conical structures called shatter cones, which point toward the center of the impact. Other evidence includes glassy, previously molten material formed at high temperature, and minerals with a deformed crystal structure produced by a shattering, high-pressure impact. Roughly ten percent of the craters also contain meteorites that had to come from space.